Congressional endorsements of presidential candidates aren’t much of what they used to be — door-openers to the sort of local organizational muscle and fat checkbooks that would scare away rivals early on or change the late dynamic of a close primary.

The televised, telemarketed and tweeted world of modern national campaigns doesn’t have much room for a regional or even statewide power broker to make a mark. And, for most senators or House members, the loss of face from embracing the loser is a much bigger worry than the all-too-often ephemeral rewards from standing in the reflected glow of a winner.

Those dynamics help explain why, by the time former Massachusetts Gov. Mitt Romney wrapped up the topsy-turvy Republican nomination contest last April, he’d been formally endorsed by only 25 senators (slightly more than half) and just 79 House members (slightly fewer than one-third).

Six Januaries ago, McCaskill was the first woman in the Senate to back her colleague Barack Obama of Illinois over her other colleague Hillary Rodham Clinton of New York. This time, she’s become the first member of Congress whatsoever to endorse Clinton — who insists she won’t reveal whether she’ll be a candidate before the end of the year.

"Hillary Clinton had to give up her political operation while she was making us proud, representing us around the world as an incredible secretary of State, and that's why Ready for Hillary is so critical," McCaskill said in a statement released by that super PAC, created this winter by a couple of lower-level 2008 Clinton campaign aides in an effort to centralize the recruiting efforts. “It's important that we start early, building a grassroots army from the ground up, and effectively using the tools of the Internet — all things that President Obama did so successfully — so that if Hillary does decide to run, we'll be ready to help her win.”

Taken in their own contexts, both early adopter moves make about equal amounts of sense for McCaskill. The first time, she still owed a significant debt of gratitude to Obama after only a year in her Senate seat, which she’d won in an upset aided by several campaign visits from the freshman senator from across the Mississippi. They had worked genuinely closely to engineer some changes to Senate ethics rules resisted a bit by their own leaders.

And McCaskill had already burned a couple of bridges with the Clintons. In 2005, she allowed that the 2008 gubernatorial race she was then planning would be dead on arrival if Hillary Rodham Clinton topped the Democratic ballot that year. In 2006, she apologized after saying on national TV that, while Bill Clinton may have been a great president, "I don't want him near my daughter."

Announcing her 2016 choice before anyone else at the Capitol affords McCaskill an opportunity to atone for those slights, if not walk away for all the valuable surrogacy work she did in Obama’s first campaign. It allows her to portray herself as at the vanguard of the 16 Democratic women in the Senate, and the 62 more in the House — the overwhelming majority of whom could be counted on to back an official Clinton candidacy. Three years from now, there’s a chance Missouri’s 10 electoral votes could be in play the way they were in 2008 if not in 2012, when the biggest solid the president did for McCaskill was to concede the state early so as not to act as a drag on her candidacy.

And, if the thickest glass ceiling in American public life is pierced, McCaskill would be in as good a position as anyone to seek a Cabinet position in 2017, when she will be only 63 but would otherwise be needing to make up her mind about committing to a surely bruising and expensive campaign for a third Senate term.

But what if she’s wrong? It’s still theoretically possible that the TBD at the end of Clinton’s brand new Twitter bio is a head fake, and the most important person who decides she’s not ready for Hillary is Hillary herself.

More likely is a situation in which Clinton reignites her ambitions to return to the White House as principal tenant but does so at a less-than-ideal time — too early next year, say, to keep the donors and professionals who might want somebody else in the deep freeze. The Democratic Party has not cleared the field for a non-incumbent presidential candidate in this lifetime, nor does it have anything like the Republican Party’s track record for awarding the nomination to the candidate perceived as next in line because of coming in second last time. (Romney, John McCain, Bob Dole and Ronald Reagan are but the most recent examples.) There’s surely some reason to be against both those precedents getting broken at once.

And then there are two even less flattering precedents to keep in mind. Clinton would be the first former secretary of State elected president since James Buchanan, and the first former senator since Richard Nixon.

Conventional wisdom is that McCaskill has become a footnote to history as the first in Congress to lay her hands on the Clinton coronation tiara. The lessons of history almost promise it will prove more complicated than that.